The present invention relates to a cathode ray tube comprising a plurality of support members for suspending a mask frame on a face panel.
In general, a cathode ray tube comprises a panel having a substantially rectangular skirt portion standing on the periphery of a substantially rectangular effective surface, a funnel connected with the skirt portion of the panel, a phosphor screen formed inside an effective surface of the panel, a shadow mask having a substantially rectangular mask body where a number of electron beam apertures are formed and a substantially rectangular mask frame attached on the periphery of the mask body, a plurality of support member for elastically suspending the shadow mask on the panel such that the mask body is opposed to the phosphor screen, an electron gun provided in a neck portion of the funnel to emit an electron beam to the phosphor screen through electron beam apertures of the mask body, and a deflector for generating a magnetic field to deflect the electron beam emitted from the electron gun.
Thus, the electron beam emitted from the electron gun is deflected by the deflector and the phosphor screen is scanned in the horizontal and vertical directions through the shadow mask, thereby displaying a color image through the panel.
A holder 40 having a substantial wedge-like shape as shown in FIG. 11 is known as a support member for supporting the shadow mask. The holder 40 has a side fixed on the mask frame 42 of the shadow mask and another side detachably engaged on a stud pin provided so as to project from the skirt portion 44.
To display an image without a color drift on the phosphor screen 46, the electron beam passing through an electron beam aperture in the shadow mask must be landed correctly on a predetermined position on the phosphor screen 46. Therefore, the positional relationship between the panel and the shadow mask, and particularly, the distance between the inner surface of the panel where the phosphor screen 46 is formed and the mask body where a number of electron beam apertures 45 are formed must be maintained with high precision.
However, if the mask body is made of a thin carbon steel plate or the like, the mask body is thermally expanded by the collision of the electron beam during operation for a long time, toward the phosphor screen 46, i.e., so-called doming is caused. When the mask body thus causes doming, the distance therefrom to the inner surface of the panel is changed and the landing of the electron beam is misregistered, thereby causing a color drift in the display image.
Therefore, the color drift caused by the doming of the mask body is compensated by using the substantially wedge-like holder 40 as described above. This means that the holder 40 is deformed as indicated by a one-dot chain line in the figure when doming occurs in the mask body and the mask body is pushed up in the direction toward the phosphor screen 46. In this manner, the landing position of the electron beam is compensated so that the position is not changed before and after the thermal expansion of the shadow mask.
In addition, as for a cathode ray tube of a relatively large size, it is known to form the mask body from an invar material having a lower thermal expansion coefficient than the mask frame. Thus, in a cathode ray tube having a mask body having a lower thermal expansion coefficient than the mask frame, the holder 50 as shown in FIG. 12 is used.
The holder 50 has a first member fixed to the mask frame 52 of the shadow mask, and a second member engaged on the stud pin 53 projected from the skirt portion 54 of the panel. The holder 50 is formed to have a substantially V-shaped cross-section which is symmetrical in the lateral direction.
If this kind of cathode ray tube is operated for a long time, the mask body is little thermally expanded but only the mask frame 52 is thermally expanded as indicated by a one-dot chain line in the figure. In this time, the shadow mask is not moved in a direction in which the shadow mask is apart from and close to the phosphor screen 56 since the holder 50 is formed to be symmetrical in the lateral direction. This means that the landing position of the electron beam can be maintained at a correct position even if the mask frame is thermally expanded.
However, even if the counter measure as described above is taken, in most cases of recent cathode ray tubes having a relatively large deflection angle, a color drift caused by misregistration of the landing position of the electron beam. To compensate such electron beam landing misregistration, the mask frame 52 needs to be moved in a direction in which the frame 52 is apart from the phosphor screen 56, i.e., toward the electron gun. Specifically, to cancel heat expansion of the mask body toward the phosphor screen 56, deformation of the holder due to thermal expansion of the mask frame must cause the mask frame to move toward the electron gun.
For example, Japanese Patent Application KOKAI Publication No. 1-14851 discloses an example of a technique of moving the mask frame toward the electron gun. Disclosed in this publication is a holder having a first member which is engaged on a skirt portion of a panel and has a greater plate-thickness than a second member fixed on the mask frame. If the plate-thickness of the first member is thus thickened, the first member tends to be deformed less easily than the second member. As a result, when the mask frame is thermally expanded, the second member is more deformed than the first member, so that the mask frame is moved toward the electron gun.
However, in this holder, a stress is concentrated on the second member having a smaller thickness than the first member when an undesired impact is applied from the outside of the cathode ray tube. Consequently, the second member which is relatively weak and tends to be easily deformed is plastically deformed and misregistered landing is caused due to the deformation of the second member, if the impact from the outside cannot be absorbed.
As a technique for preventing misregistered landing due to an external impact, Japanese Patent Application KOKOKU Publication No. 64-27144 proposes a holder 40 having bending portions 40a and 40b. The bending portions 40a and 40b function to restrict a movement of the mask frame against an external impact from a direction vertical to the page surface in FIG. 11, but is not effective against an external impact from the direction of the tube axis of the cathode ray tube.
Also discussed is a method of improving the rigidity of respective components of the holder to hinder plastic deformation. However, if this method is adopted, it is not possible to correct sufficiently a color drift caused under influences from a heat when the cathode ray tube is operated for a long time, but also the detachability of the holder during manufacturing steps is degraded.
As described above, the shape of the holder holding the shadow mask is important to display an image on a phosphor screen of a cathode ray tube without a color drift. However, it is difficult for conventional techniques to provide a holder which is capable of simultaneously correcting both a color drift caused by a change of the positional relationship between the shadow mask and the panel due to an external impact and a color drift caused due to influences from a heat when the cathode ray tube is operated for a long time.